the peripheral auditory system george pollak section of neurobiology
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The Peripheral Auditory System
George PollakSection of Neurobiology
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Organ of Corti
Basilar membrane
• Hair cells, the transducers of the auditory system, and how they work.
1
stereocillia ofinner hair cells
stereocillia ofouter hair cells
stereocilia on one hair cell
9
a
trap doorofionicchannel
tip linkstereocilia
A B C D
GE F
A model for mechoelectrical transduction by hair cells. In the absence ofany stimulation, at any instant each transduction channel at a stereocilium'stip may be either closed (A and F) or open (E). The greater probability isthat the channel is closed. When the hair bundle is deflected with a positivestimulus, the spring or tip link, exerts a force on the trap door and opensthe channel (B,C and D). The influx of K+ ions into the hair cell causesit to depolarize. Pushing the hair bundle in the opposite direction compressesthe spring (tip link), ensuring that the channel remains closed (G). Thisprevents the influx of K+ ions and causes the cell to hyperpolarize.
aa
Endolymph
PerilymphFig. 1
apical surfaceof hair cell
basal surfaceof hair cell
stereociliakinocilium
tight junctionssynaptic ribbonsafferent nerves
Hi K+
Lo Na+
Hi Na+
Lo K+
Hi K+
Lo Na+
Ek= 58 logKout
Kin
= 0mV
Potential difference betweenEndolymph and cell interior
Ek= 58 logKout
Kin
= ~-70mV
Potential difference betweenPerilymph and cell interior
endolymph
perilymph
aaa
K+
K+
Ca++ K+
InhibitionK+
K+Ca++
ExcitationRest
Rest: Small amount of K+ leaks into cell from rattling channels of stereocilia. The leakingK+ depolarizes cell thereby opening voltage sensitive Ca++ channels resulting in spontaneousrelease of transmitter and excitation of afferent nerve fibers (not shown). The depolarizationcauses K+ to leave cell in basal region.Excitation: Stereocilia are bent thereby opening channels which results in larger entry of K+into cell. The influx of K+ causes additional depolarization thereby opening more voltage gatedCa++ channels. The additional Ca++ channels that are now open cause a larger influx of Ca++in the base. The influx of Ca++ causes more transmitter to be released and thus a greaterexcitation of the afferent fibers (not shown).Inhibition: Stereocilia are bent in opposite direction thereby closing channels. This results in less influx of K+ at channels in the tips of stereocilia. K+ efflux occurs at base and is notreplenished by influx at stereocilia. Consequently, the hair cell hyperpolarizes thereby closingvoltage sensitive Ca++ channels at the base resulting in a smaller release, or even no release oftransmitter. Discharge of afferent fiber is therefore reduced to a rate even below the restinglevel.
-45 mV
Hi K+
low Na+
Hi K+
low Na+
Hi K+
low Na+Hi Na+
low K+
Hi Na+
low K+
small leakage of K+ into cell
-45 mV
K+ into cell
-70 mV
No K+ into cell
Next, we are going to build a cochlea
Basilar membrane
Stapes
Sound is changed froma pressure wave in the air
into mechanical movements on the basilar membrane
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round window
Traveling waves on basilar membrane
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oval window
The structure of the basilar membrane causes it
to perform a frequency to place transformation
Basilar Membrane
has continuously changing dimensions along its length
Baseresponds maximally to high frequencies
Apexresponds maximally to low frequencies
StiffNarrow and thick
flexiblewide and thin
Basilar membrane converts frequency to a place of maximal response
Frequency-to-Place Transformation in the Cochlea
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The motion on the basilar membrane
causes shearing of the cilia onhair cells and thereby causes
the hair cells to depolarize and hyperpolarize in
response to sound
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Organ of Corti
Basilar membrane
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Organ of Corti
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basilar membranebasilar membrane
shearing of stereocillia
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Why are there two types of hair cells?
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98% of the fibers that project into the central auditory systemare innervated by inner hair cells!!
98%
What are the outer hairs doing?Answer: they act as amplifiersof the mechanical motion of
the basilar membrane generatedby sound
hyperpolarization-----
depolarization
release of transmitter
Hi K+K+
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Evoked mechanical responses of isolated cochlear outer hair cells.
Electromotility: OHC can change length in response to voltage change
Direct evidence of an active mechanical process in the organ of Corti
depolarized hyperpolarized
+++
___
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42
Dancing hair cell
Outer hair cells are the only cells in the body that express prestin. Even inner hair cells do NOT have prestin.
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Show movie of how Prestin works
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Sound stimuli
Basilar membrane
motion
Hair bundle deflection
Membrane potential change
Change in length of hair cells
IHC Sensory signal transmission
OHC
Positive feedback loop
Normal response with cochlear
amplifier
response without cochlear
amplifier
base
Apex
base
Apex
Basilar membrane
How motion of basilar membranegenerates tuning curves in
auditory nerve fibers and therebyimparts frequency selectivity
to auditory nerve fibers
6 kHz
7 kHz
8 kHz
9 kHz
10 kHz
10
6 7 8 9 10 11
Frequency ( kHz)
5
20
30
40
50
60
Inte
nsi
ty (
dB
SP
L)
base apex
50 dB SPL
6 kHz
7 kHz
8 kHz
9 kHz
10 kHz
10
6 7 8 9 10 11
Frequency ( kHz)
5
20
30
40
50
60
Inte
nsi
ty (
dB
SP
L)
Tuning Curve The most basic feature of an auditory neuron
best frequency
30 dB SPL
base apex
frequencylow high
Sou
nd in
tens
ity
low
high
tuning curves in normal animalstuning curves in animals with no outer hair cells or in animals without prestin gene
How is the tonotopic organizationthat was first established on the
basilar membranepreserved in in the
central auditory system?
Inferior
colliculus
Inferior
colliculus
Medial geniculate
Medial geniculate
Superior olive
Superior olive
Cochlear nucleus
Cochlear nucleus
Auditory cortex
Cochlea
Cochl
ea
Auditory nerve
Auditory nerve
Flow of Information Along the Central Auditory Pathway
Inferior
colliculus
Inferior
colliculus
Medial geniculate
Medial geniculate
Superior olive
Superior olive
Cochlear nucleus
Cochlear nucleus
Auditory cortex
Cochlea
Cochl
ea
Auditory nerve
Auditory nerve
cochlear nucleuscochlear nucleus superior olive
Inferior colliculus Inferior colliculus
medial geniculate medial geniculate
auditory cortex
The Frequency Representation on the Cochlea is Preserved in Every Nucleus of the Central Auditory System, and thus the Auditory
System is Tonotopically Organized
Inferior
colliculus
Inferior
colliculus
Medial geniculate
Medial geniculate
Superior olive
Superior olive
Cochlear nucleus
Cochlear nucleus
Auditory cortex
Cochlea
Cochl
ea
Auditory nerve
Auditory nerve
The Frequency Representation on the Cochlea is Preserved in Every Nucleus of the Central Auditory System, and thus the Auditory
System is Tonotopically Organized
The Frequency Representation on the Cochlea is Preserved in Every Nucleus of the Central Auditory System, and thus the Auditory System is Tonotopically Organized
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